β-type zeolite is known as a zeolite which is used for a NOx reducing catalyst, in particular a NOx reducing catalyst using ammonia as a reducing agent (in general, called “SCR (an abbreviation of selective catalytic reduction) catalyst”) (see Patent Document 1).
The SCR catalyst using β-type zeolite could not be used under a condition of low exhaust gas temperature because after an endurance treatment under a high-temperature water vapor atmosphere (hereinafter referred to as “hydrothermal endurance treatment”), the NOx reducing performance is lowered, in particular the deterioration of activity at a low temperature of not higher than 300° C. is large. Though it may be considered that the lowering of performances of the SCR catalyst using such β-type zeolite after the hydrothermal endurance treatment is mainly caused due to insufficient hydrothermal resistance of β-type zeolite, in particular a cause of lowering of the low-temperature activity has not been sufficiently elucidated yet. In any of performances of SCR catalysts reported by conventional documents, or performances of SCR catalysts using commercially available β-type zeolite, a NOx reduction rate at 200° C. after hydrothermal endurance treatment was less than 45%.
β-type zeolite is well known as a zeolite which is used for a catalyst or an adsorption agent, and as a method of enhancing its hydrothermal resistance, it is known to increase an SiO2/Al2O3 molar ratio, or to make a crystal size large (see Patent Documents 2 and 3). But, in the case of making the SiO2/Al2O3 molar ratio large, a solid acid, namely a catalytic activity point becomes few; whereas in the case of increasing the crystal size, a diffusion rate in the catalyst is lowered, and such is disadvantageous for catalyst properties under a transitional condition under which the temperature changes with time, so that sufficient dissolutions have not been attained yet in the application of SCR catalyst.
There have hitherto been made some proposals regarding an enhancement of performances of the SCR catalyst using β-type zeolite. For example, there is proposed a method of hydrothermally treating the SCR catalyst using β-type zeolite prior to the use, thereby achieving a treatment (Patent Document 4). But, performances after the endurance treatment were not sufficient yet. Also, there is reported a method of further adding a rare earth element to iron which is usually used as a supported metal of catalyst, thereby enhancing the hydrothermal resistance (see Patent Document 5). But, even in the subject method, nevertheless the expensive rare earth element is necessary, the NOx reduction rate at 200° C. was still less than 45%.
Also, there is proposed a technique for enhancing the endurance post-activity by an SCR catalyst in which an SiO2/Al2O3 ratio, an SEM particle size and an NH3 adsorption of β-type zeolite have been optimized (Patent Document 6). But, the NOx reduction rate at 200° C. was still less than 45%.